1. Field of Invention
The present invention relates to an expandable endoprosthesis that is placed within a tubular member of the human body to treat a region that is pathologically affected by supporting it or holding the tubular member outwards. More specifically the invention relates to an intravascular endoprosthesis placed within a blood vessel of the body generally at the site of a vessel lesion in order to provide a more widely open lumen and enhance patency of the vessel. The present invention further relates to a stent that can be used in blood vessels and tubular vessels of the body that have become stenotic or blocked by tissue or other material and require reestablishment of a lumen and maintenance of the lumen.
2. Description of Prior Art
Stents used to internally support tubular vessels of the body can generally be categorized into two groups, those that are mechanically expanded by an external device such as a balloon dilitation catheter, and those that are self-expandable. Advantages of the balloon expandable stents lies in part in the ability of these stents to be delivered accurately to the site of a stenotic lesion. The location of the stent prior to deployment or expansion can be visualized under flouroscopy and deployment of the stent is generally made by inflation of a dilitation balloon which expands the stent radially into contact with the inner surface of the vessel wall. After the dilitation balloon has been deflated and the dilitation catheter removed, the stent is left in place to balance radial forces applied by the vessel wall and ensure that the vessel lumen is maintained in a widely patent conformation.
Some difficulties associated with balloon expandable stents can be related to their lack of flexibility and their inability to withstand external forces that can lead to irreversible crushing of the stent. Several stents exhibit a structure that will not easily bend around tortuous pathways found in the human vasculature to reach the site of the lesion in their nondeployed state. Other stents have a structure that is made more flexible but are not appropriately capable of supporting or balancing the radial forces applied by the vessel acting to compress the stent. A stent with a low radial balancing force characteristics may be expected to undergo an irreversible crushing action if exposed to an externally applied force. Arteries of the neck and leg region can sometimes be exposed to such external forces resulting in permanent deformation of the stent and loss of vessel patency. This has been the case for some balloon expandable stents that have been placed in the carotid artery and exposed to digital or other external forces that have led to collapse of the stent.
In the balloon expandable stents currently found in the prior art one cannot adjust the amount of force required to expand the stent independently from the force required to crush the stent. As a result, a stent design that resists crushing action will generally be too stiff and require too much force to accomplish its deployment.
Self-expandable stents overcome some of the problems associated with the crushability of balloon expanded stents. These stents are typically made of Nitinol, a stainless steel with high yield strength, or some other material that can store energy elastically. Self-expandable stents can be delivered within a sheath to the site of the lesion. There the sheath can be removed and the stent can be deployed as it expands out to a larger diameter associated with its equilibrium diameter.
Some problems associated with self-expandable stents include the inability of the physician delivering the stent to define precisely the location of both ends of the stent. Oftentimes the stent can undergo significant changes in it axial length in going from an nondeployed state to a deployed state. Such length changes can result in inaccuracies in defining a precise placement for the stent. This disadvantage can be somewhat offset by the ability of some self-expandable stents to resist crushing deformation associated with an external force directed toward the side of the stent. Self-expandable stents also do not generally allow the radial expansion force to be adjusted independently from the stent forces that are directed to resist crushing forces. A self-expandable stent with an appropriate elastic balancing force to hold the vessel open may have a weak crush balancing force to resist crushing deformation due to externally applied crushing forces.
Palmaz discloses in U.S. Pat. No. 4,733,665 a balloon-expandable stent that is formed by machining slots into a metal tube forming a series of elongate members and bars. The stent is mounted in its nondeployed state onto the balloon portion of a balloon dilitation catheter and delivered to the site of a lesion that has been previously dilated to allow passage of the stent mounted balloon catheter. Dilation of the balloon causes the balloon-expandable stent to plastically deform at the junction of the elongate members and bars of the stent. For the stent to undergo an expansional deformation the balloon must supply an expansion applied force that exceeds the expansion yield force associated with the junction of the elongate members and the bars. Typically a balloon dilitation catheter used to dilate a coronary lesion found in a three millimeter diameter coronary artery can be dilated at a balloon pressure ranging from one to fifteen atmospheres. With the stent mounted on the balloon, the balloon must be capable of radially expanding the stent and holding the vessel in a widely patent conformation. Upon removal of the balloon catheter, the stent must continue to supply a compression balancing force to balance the compression applied force of the vessel acting inward on the stent. If an externally placed side force is imposed onto the side of the stent, the stent can deform into an oval or flattened shape representative of a crushing deformation. This deformation can involve an elongate member or it can occur at a junction of an elongate member with a bar. The elongate member can be formed such that it resists plastic deformation associated with crushing deformation. The junction of the elongate member with the bar cannot be adjusted to resist crushing deformation without also affecting the force required to expand the stent from its nondeployed state to its deployed state; additionally, the compression balancing force would also be affected. The Palmaz stent disclosed herein therefore can be susceptible to crush deformation in order to maintain appropriate characteristics for an expansion yield force during deployment and a compression yield force to hold the vessel in an open conformation.
A stent is required to have axial flexibility in order to negotiate the tortuous turns found in the coronary vasculature. Palmaz describes in U.S. Pat. No. 5,102,417 connector members that connect between small cylindrical stent segments. Although the connector members provide an enhanced axial flexibility, this stent is still subject to crush deformation. The compression yield force that is capable of holding the vessel outward with the stent in a deployed state is coupled to the crush yield force that prevents the stent from crush deformation.
Fischell describes in U.S. Pat. No. 5,695,516 a balloon-expandable stent formed from a metal tube and having circumferential arcs and diagonal struts. When this stent is expanded to a deployed state the junctions of the arcs and struts undergo plastic deformation and the deployed stent takes on a honeycomb shape. The expansion yield force of this stent describes the force required to plastically deform an arc with respect to a strut at a junction during stent expansion. The compression yield force describes the force required to plastically deform an arc with respect to a strut when exposed to a compression applied force by the blood vessel. Deformation due to crushing would also occur at the junction of the arc with the strut. The crush yield force of this stent is therefore directly coupled to the expansion yield force and the compression yield force. If this stent were designed to expand upon exposure to a dilitation balloon with a nominal expansion applied force, it would not be able to resist crushing deformation when exposed to an external side force that can be encountered in a carotid or femoral artery position.
Fischell describes in U.S. Pat. No. 5,679,971 a balloon-expandable stent that has two different types of cells, one for radial rigidity and one for axial flexibility. Neither one of these cells addresses the need to provide an anti-crush characteristic to the stent. One cannot adjust the crush yield strength of this stent independently of the expansion yield strength.
Lam (U.S. Pat. No. 5,649,952), Anderson (U.S. Pat. No. 5,800,526), Frantzen (U.S. Pat. No. 5,843,164), and Orth (U.S. Pat. No. 5,681,346) each describe balloon-expandable stents formed from metal cylinders and having serpentine wave patterns connected by interconnecting members. In each of these stent disclosures the stent has a crush yield force that is coupled to the expansion and compression yield force. These stents would be susceptible to irreversible crush deformation if exposed to an external force to the side of the stent.
Gianturco discloses in U.S. Pat. No. 4,800,882 a balloon-expandable stent constructed from a metal wire and discloses in U.S. Pat. No. 5,041,126 a method of insertion for a balloon-expandable stent. The wire stent disclosed by Gianturco has adjacent curved sections or loops joined by a bend or cusp. During stent expansion by a balloon the loops diverge as the metal wire irreversibly and plastically deforms. The deformation of the metal wire that occurs during expansion has a similar yield force as the deformation of the metal wire that can occur during compression of the stent if the compression applied force of the vessel exceeds the compression yield strength of the stent. If this stent is exposed to an external side force that could lead to a crush deformation, this stent can undergo plastic deformation that is irreversible and can occur with a similar yield force as the compression yield force. This stent is not well suited to provide independent adjustment of compression yield force with respect to crush yield force. As a result, this stent can be subject to crushing deformation even though the expansion and compression yield force are appropriate for allowing balloon expansion and support of the blood vessel.
Hillstead (U.S. Pat. No. 4,856,516), Wiktor (U.S. Pat. Nos. 5,133,732 and 4,886,062), Globerman (U.S. Pat. No. 5,776,161), Fontaine (U.S. Pat. No. 5,527,354), Horn (U.S. Pat. No. 5,591,230), Boyle (U.S. Pat. Nos. 5,613,981 and 5,591,198), and Hillstead (U.S. Pat. No. 5,116,365) each describe balloon-expandable wire stents made from loops, zig zags, helical wires, curved rings, sinusoidal waves, or other similar form of construction. All of the stents described in these disclosures are expandable by the expansion applied forces of a balloon of a balloon dilitation catheter or other similar catheter. During the expansion of the stents, the wires or each stent undergoes a plastic deformation once the expansion yield force has been exceeded. A plastic deformation would also be required for any of these stents to compress under the compression applied force applied by the vessel wall; this could occur once the compression yield force of the stent has been exceeded. Exposure of any of these stents to an external side force could lead to a crush deformation. It is not possible for any of these stents to enhance the crush yield force without altering the expansion or compression yield force of the stent. Cox (U.S. Pat. No. 5,733,330) and Wall (U.S. Pat. No. 5,192,307) each describe balloon-expandable stents with ratchet mechanisms. The stents can be formed out of an elastic metal that will not allow for stent crushing and the ratchet mechanism can prevent the stents from collapsing under the force of compression applied by the blood vessel. Each one of these disclosures describes a separate latching or ratcheting mechanism that is required to provide the properties of balloon-expandable and non-crushability. The latching or ratcheting mechanism adds to the size and the complexity of the device.
Wallsten describes in U.S. Pat. Nos. 4,655,771 and 5,061,275 self-expandable stents formed from helically wound braided flexible thread elements or wires. The metal wires are elastic or resilient in nature with a high energy storage capacity. The stents can be delivered to the site of the lesion by an external sheath that applies a constraining force upon the stent to hold it in an nondeployed state of a smaller diameter. Upon release of the stent within the blood vessel, the stent undergoes a radial expansion to a larger predetermined diameter. The vessel provides a compression applied force due to vessel elasticity and collagenous scarring and contraction that can occur during vessel healing; this compression applied force acts inward on the stent. The stent in its deployed state provides an expansion elastic balancing force outward against the vessel wall to balance the compression applied force. If the stents described by these disclosures are acted upon by an external crush applied force delivered to the side of the stent, the stents can undergo an crush deformation forming an oval or flattened shape. The stents can provide a crush elastic balancing force to balance the crush applied force and limit the amount of crush deformation. The degree of crush deformation that can occur for a specific crush applied force is directly related to the size and number of flexible thread elements or wires used in the formation of the stents. The size and number of wires has a direct bearing on the expansion elastic balancing force provided by the stents. Therefore an increase in the crush balancing force will generally be associated with a corresponding increase in elastic balancing force. It can be desirable to adjust the crush balancing force independently of the elastic balancing force. The stents disclosed by Wallsten are not well suited to independent adjustment of the crush and the expansion elastic balancing force. Similarly, the crush balancing force is directly coupled to the expansion elastic balancing force.
Gianturco describes in U.S. Pat. Nos. 4,580,568 and 5,035,706 self-expandable stents formed from metal wire in a zig-zag pattern. These stents are elastically compressed to a smaller diameter for delivery within a blood vessel and undergo an elastic expansional deformation during delivery at the lesion site. The stents provide a deployed expansion elastic balancing force outward against the vessel wall to maintain the diameter of the vessel in an open and widely patent conformation. If exposed to an external crush applied force the stents will deform elastically and provide a crush elastic balancing force. The expansion and crush elastic balancing force are directly coupled and are not easily varied with respect to one another. Such stents with appropriate expansion characteristics are not easily adjusted to provide altered crush characteristics independently of one another.
Lauterjung (U.S. Pat. No. 5,630,829) and An (U.S. Pat. No. 5,545,211) describe self-expandable stents formed from metal wire. Lauterjung provides a high hoop strength stent due to the angle of the wire in the expanded state. An describes a zig-zag pattern that is spiraled into turns and is cross-linked with each other at adjacent turns. Each of these stents has an expansion and a crush elastic balancing force that is coupled directly to each other. One can not appropriately adjust the expansion characteristics with respect to the crush characteristics.
Carpenter describes in U.S. Pat. No. 5,643,314 made of a series of elastic metal bands or loops interconnected along a backbone. A lock is used to hold the loops in a contracted configuration around a balloon portion of a delivery catheter during delivery to the lesion site. Once expanded by the balloon, a lock is used to hold the loops outward in their expanded configuration. The strength of the lock provides the balancing force of the stent to hold the vessel in an open widely patent configuration. The dimensions of the metal loops determines the crush balancing force for a specific crush deformation. This stent is cumbersome to use with sliding required between metal and a locking mechanism that occupies areal space and volume.
McIntyre (U.S. Pat. No. 5,833,707) and McDonald (U.S. Pat. No. 5,728,150) each describe a stent formed from a flexible elastic metal sheet that has been coiled into a small diameter for delivery to a blood vessel. Upon release of the coiled stent, it springs out to form a larger deployed diameter and hold the vessel with an expansion elastic balancing force. The crush balancing force of these stents involves a similar defomation of the metal sheet as the expansion or compression deformation involved with the expansion delivery of the stents or collapse of the stents due to vessel compression applied forces. These stents do not provide for adjustment of the crush balancing force with respect to the expansion or compression balancing force.
Dotter (U.S. Pat. No. 4,503,569), Alfidi (U.S. Pat. No. 3,868,956), and Froix (U.S. Pat. No. 5,607,467) describe coiled stents constructed out of plastic or metal that can change in shape from a small diameter to a large diameter due to the application of heat, or application of another external condition. Most plastic stents are not acceptable due to the inadequate strength per volume of material in comparison to a metal stent. As a result, plastic stents require excessive areal space or volume which can be very undesirable in a small diameter blood vessel. Metal stents with a coiled shape have a similar mode of deformation in providing an expansion or compression balancing force in comparison to providing a crush balancing force. These stents do not allow the crush balancing force to be adjusted with respect to the expansion or compression balancing force.
Roubin discloses in U.S. Pat. No. 5,827,321 a stent that is radially expandable by balloon or self-expandable and designed to maintain its axial length upon expansion. The stent has annular elements connected by connecting members. The connecting members are formed from Nitinol and have a desire to lengthen upon deployment of the stent. The expansion or compression balancing force against the vessel wall is provided by the annular elements which have a curved or zig-zag structure. Upon exposure to a crush deformation it is the annular elements that provide the crush balancing force. The crush balancing force is coupled to the expansion or compression balancing force; the stent does not provide for independent adjustment of the balancing forces.
Williams (U.S. Pat. No. 5,827,322) describes a balloon-expandable or self-expandable stent formed from Nitinol flat metal sheet and having a ratchet mechanism to hold the stent in an expanded state. This stent is not flexible in the axial direction and the ratchet mechanism requires additional areal space and volume. This stent does not allow independent adjustment of crush balancing force without also significantly impacting the expansion elastic balancing force provided by the stent.
Hilaire describes in International Application with International Publication Number WO 98/58600 an expandable stent with variable thickness. The variable thickness is intended to allow the balloon expandable stent to expand more evenly along its perimeter. The stent is formed from a plurality of tubular elements with a zig zag shape that are joined together by linking members. This device does not teach or describe a stent that provides independent adjustment of stent expansion forces with respect to stent crush forces.
The present radially expandable intravascular stent overcomes the disadvantages described for other prior art balloon-expandable and self-expandable stents. The stent of the present invention has nodes that are attached to two or more struts. Each node has a hinge that focuses the deformation associated with expansion of the stent from its nondeployed or insertion state with a smaller diameter to its deployed or implanted state with a larger diameter. The radially expandable stent of this invention exerts forces against its environment and is exposed to applied forces from the environment. These forces will be described by the element generating the force, the direction of the force in either expansion or compression, and the type of force being exerted. For a balloon-expandable stent the hinge allows an inserted stent compression yield force and an implanted stent expansion yield force to be decoupled from an implanted stent crush elastic force thereby providing a stent that can be balloon expandable but non-crushable based on strut and hinge dimensions. This can be extremely valuable for applications such as carotid stenting where exact placement of a balloon-expandable stent is critical and the ability of the stent to resist crushing is a necessity. For a self-expandable stent, decoupling an implanted stent crush elastic force from an implanted stent expansion elastic force provides a stent that can be soft in crush deformation but provide adequate expansion elastic force to hold the vessel open. This type of stent may be advantageous in specific coronary artery stenting applications. Alternately, a self-expandable stent can be formed under the present disclosure that provides a large implanted stent crush elastic force but with a small or modest implanted stent expansion elastic force. This type of device may be useful in a situation where scar tissue may be contracting down on a vessel lumen. The stent of the present invention can be applied to any tubular vessel or passage found in the human body. Application of the hinge stent of the present invention can be made in particular to treatment of arterial blood vessels of the body. Such arterial blood vessels that can be treated with the present invention include small arteries such as coronary arteries and carotid arteries, middle size arteries such as femoral arteries and other vessels of the leg, and larger vessels including the aorta.
For a radially expandable balloon-expandable stent, a balloon expansion applied force is applied to the inside surface of the stent by a balloon of a balloon dilitation catheter or a similar catheter. Dilation of the balloon causes the stent to undergo a radial expansion that requires a plastic deformation such that the stent exceeds its elastic limit and exceeds the inserted stent compression yield force. A typical balloon dilitation pressure for a three millimeter diameter balloon is about 5-10 atmospheres. Balloon pressures can range from one atmosphere to dilate a very soft lesion to over 15 atmospheres to dilate a heavily calcified lesion. The stent will retain its expanded or deployed diameter and a tissue compression applied force applied by the blood vessel would have to exceed the implanted stent expansion yield force for the stent to collapse. The force exerted by the stent on the vessel wall is an implanted stent expansion holding force that must be large enough to hold the vessel at the deployed or implanted diameter of the stent and resist any diameter changes due to external forces or scarring. The inserted stent compression yield force cannot be so high that the balloon catheter is unable to expand the stent to the appropriate diameter. During this radial expansion of the stent the most significant plastic deformation occurs as an expansion deformation in an axial or circumferential direction within the uniformly curved surface of the stent, and relatively little crush deformation occurs with respect to the radius of curvature of the stent within a cross section. If a deployed stent is exposed to an external tissue crush applied force from one side it tends to form an oval shape representative of a crush deformation. If the tissue crush applied force exceeds the implanted stent crush yield force, the oval shape becomes extreme and can flatten as the stent can exceed its elastic limit in a crush deformation. During the crush deformation the wall of the stent is formed into an oval shape that is not the same as the expansion deformation encountered during the radial expansion.
The stent of the present invention allows a balloon-expandable and non-crushable stent to be formed from a metal that has a relatively high yield strength or high expansion yield force. The yield strength for the metal of the stent can be high enough such that when the stent is formed into an oval or flattened shape such as that found during crush deformation, the struts do not surpass their elastic limit and hence remain elastic. Each strut can be connected to two nodes through a hinge. The hinge is configured to provide plastic expansion deformation to the stent as it extends from a nondeployed insertion diameter to a deployed implantation diameter. The expansion deformation is focused in the hinge region such that localized plastic deformation occurs in the hinge. The hinge resists bending in the radial direction such as that deformation produced during a crush deformation. The result is a stent with appropriate inserted stent compression yield force that can be properly overcome by the balloon expansion applied force from the balloon dilitation catheter and strength to support the blood vessel and resist vessel contraction, and further provide the stent with non-crush characteristics.
A balloon-expandable stent of one embodiment of the present invention can be modified to provide modified stent expansion and compression yield force characteristics or modified implanted stent crush yield force characteristics independent of one another. This is accomplished by altering a radial dimension for the hinge and changing its width and length perpendicular to the radial dimension. For example, to increase the implanted stent crush yield force while maintaining the inserted stent compression yield force constant, the struts can be first enlarged in their sectional area to provide the desired crush yield force, or the material of the stent can be changed to attain a higher yield strength material. The struts can be formed of appropriate material and thickness to ensure that they remain elastic for any reasonable crush deformation encountered during normal use. The radial dimension of the hinge can be increased to provide an accompanying similar increase in crush yield force to the node. The width of the hinge in a direction perpendicular to the radial dimension and lying in the uniformly curved surface of the stent can be decreased such that the inserted stent compression yield force will remain. constant. The hinge length can be decreased to further focus the expansion deformation in the hinge and ensure that plastic deformation will occur in consideration of the narrowing of the hinge width. If the hinge length is maintained in a longer configuration, the hinge can be configured to remain in an elastic state and not undergo a plastic deformation during expansion deformation; this node and hinge design of the present stent thus allows the formation of a self-expandable stent to also be accommodated.
A self-expandable stent generally has an equilibrium diameter wherein it is not applying any radial forces; this equilibrium diameter is somewhat larger than or approximately equal to the diameter of the vessel in which it is deployed or its implantation diameter. For a radially expandable self-expandable stent, the stent can be held and delivered to the blood vessel in a nondeployed state of smaller insertion diameter with a sheath compression applied force applied by some external holding means such as a sheath. The self-expandable stent exerts an inserted stent expansion elastic force upon the external holding means. Upon release from the holding means within a blood vessel, the self-expandable stent expands, and once fully deployed exerts an outwardly directed implanted stent expansion elastic force upon the walls of the blood vessel that is dependent upon the equilibrium diameter of the stent. If the implanted stent expansion elastic force is too large when the self-expandable stent comes into contact with the blood vessel, the blood vessel wall can begin to dilate and the stent can travel into the vessel wall causing trauma. If the implanted stent expansion elastic force is too low, vessel scarring and retraction of the vessel wall can apply a tissue compression applied force on the stent causing the stent to reduce in diameter to a value smaller than desired.
As the self-expandable stent undergoes an expansion from an nondeployed or insertion diameter to a deployed or implantation diameter an elastic expansion deformation occurs within the stent. This elastic expansion deformation occurs in the axial and circumferential direction of the stent wall and is not the same as the curvature change which occurs with respect to the radius of curvature of the stent in a radial direction during stent crushing. This elastic expansion deformation is very different from the deformation that would occur if the stent were exposed to a force along its side that would cause it to form an oval sectional shape associated with a crush deformation.
The stent of the present invention allows a self-expandable stent to be formed out of an elastic metal that will not plastically deform during normal use involving elastic expansion deformation or during exposure to a crush deformation. The struts could be formed with a sectional dimension that provides the self-expandable stent with an elastic crush deformation when exposed to an external tissue crush applied force to form an oval shape due to a crush deformation. The implanted stent crush elastic force of the stent can be high or low depending upon the desired properties of the stent. The struts are connected to one or more nodes through a hinge. The hinge has a greater radial dimension than the struts to resist the formation of an oval cross section associated with crush deformation. The hinge has a width that can be adjusted to provide an implanted stent expansion elastic force that is appropriate to resist the tissue compression applied force of the blood vessel. The hinge length can further be adjusted to alter the implanted stent expansion elastic force. The result is a self-expandable stent with appropriate expansion elastic force properties and a soft feel in a crush deformation. Similarly, the stent can have appropriate or nominal expansion elastic force properties and a very rigid, difficult to crush characteristic associated with a large implanted stent crush elastic force.
A self-expandable stent of the present invention can be modified to alter the inserted stent and the implanted stent expansion elastic force or implanted stent crush elastic force independent of one another. For example, to increase the crush elastic force while maintaining the inserted or implanted expansion elastic force, the struts can be enlarged in sectional area to provide the desired crush elastic force. The radial dimension of the hinge can be increased to provide an accompanying similar increase in crush elastic force in the node region. The width of the hinge in the uniformly curved surface of the stent can be decreased such that the expansion elastic force will remain constant. The hinge length can be decreased to provide a more focused bending of the hinge through a smaller radius of curvature to generate the appropriate expansion elastic force.
In an embodiment of the present invention a metal tube constructed of stainless steel, Nitinol, titanium, tantalum, or other metal used in constructing stents is machined using laser machining, mechanical machining, chemical etching or other machining process to form raised areas in the outside surface of the metal tube. These raised areas have a greater radial dimension than the rest of the tube and will later be formed into the hinges of the stent. It is important to note that the raised areas can be formed such that their radial dimension is as thin as any normal or standard balloon-expandable or self-expandable stent. The strut region can be formed such that the radial dimension is thinner than the strut region of a standard stent. The design of the present invention allows a material of greater elastic modulus to be chosen for stent formation thereby providing appropriate expansion force characteristics based on the design of nodes and struts utilizing a thinner strut dimension than could be used with other prior art stents. Slots are then formed into the metal tube using laser, mechanical, or chemical machining methods. The slots can be any combination of straight slots or curved slots at any combination of parallel, perpendicular, or at an oblique angle with respect to the axis of the tube. The metal tube is thereby formed into a repeating array of struts and nodes with each node generally connected to at least two struts through a hinge. Each strut has a radial dimension, a width, and a length and it extends between two nodes. The nodes have a greater radial dimension than the struts. Each node can include a hub which has a radial dimension that is greater than the strut radial dimension. The hub is connected to or contiguous with at least two hinges which have a width that is narrower than the width of the struts. The node can be a single long hinge if it is connected to only two struts. Each hinge has a transition region which serves to join and provide a uniform transition between a hinge and a strut. Each transition region expandable is formed of the same metal as the hinge and the strut and is therefore contiguous with the hinge and the strut.
In one embodiment of a balloon-expandable stent the node is connected to four struts. As the slotted tubular stent is expanded from an nondeployed or insertion diameter to a deployed or implanted diameter an array of diamond shaped spacings is formed between the struts and the nodes. Upon exposure of the stent to expansion deformation, plastic deformation occurs in the hinges located between the struts and the hubs. The struts move with respect to the hub during expansion deformation of the stent such that the stent is capable of supporting a blood vessel in an expanded or deployed state. Exposure of this expanded stent to a side force tends to create a crush deformation. The entire stent and the struts are formed of a metal with a high yield strength and hence the struts will bend elastically when exposed to such a crush deformation. The node, including the hub, the hinge, and the transition region have a greater radial dimension and hence will not deform significantly under the crush deformation. Thus the stent undergoes plastic expansion deformation in a localized region of the hinge but remains elastic in all other areas to resist irreversible plastic crush deformation.
In another embodiment a self-expandable stent has each node connected to four struts and has an equilibrium diameter approximately equal to its deployed diameter. Diamond shaped spacings are found between the struts and the nodes in a deployed state. This stent can either be machined in the deployed diameter or it can be machined in the nondeployed diameter or in an intermediate diameter between the deployed diameter and the nondeployed diameter and work hardened to form an elastic self-expandable stent with an equilibrium diameter approximately equal to the deployed diameter. Prior to delivery the stent is collapsed down to a nondeployed diameter and delivered through a constraining sheath or other delivery system to the blood vessel. The nondeployed self-expandable stent exerts an inserted stent expansion elastic force outward against the sheath. Upon delivery to the blood vessel and removal of the constraining sheath the self-expandable stent attempts to assume its equilibrium diameter and hold the blood vessel in an expanded state with an implanted stent expansion elastic force. The inserted and implanted stent elastic force for a particular metal of construction is determined primarily by the length, width, and radial dimension of the hinge. Exposure of this stent to a crush deformation causes the struts to reversibly bend in the shape of the oval cross section. The strut has been designed to deform elastically when exposed to a crush deformation. The strut can either provide a large implanted stent crush elastic force when deformed to a particular degree of deformation or a small implanted stent crush elastic force dependent upon the width, length, and radial dimension of the strut. The magnitude of the implanted stent crush elastic force is independent of the expansion elastic force which is determined by the hinge width, length, and radial dimension. The self-expandable stent of this invention allows the implanted and inserted expansion elastic force to be altered independently from the implanted stent crush elastic force. The stent of the present invention can be formed of a single cylindrical stent segment or section or it can be formed of two or more stent sections joined together by a connecting means.
In further embodiments of the present invention, one or more cylindrical stent sections of either the self-expandable stent or the balloon-expandable stent formed with an array of nodes and struts can be connected together with one or more flexible connecting means. The connecting means can be a hinged interconnector formed of nodes and struts similar to those of the stent section structure or it can be a connecting element formed of a straight or curved connecting leg without nodes or hinges. Two or more stent sections can be connected together with hinged interconnectors or connecting elements to provide the stent with additional axial flexibility around a bend in a blood vessel. Axial flexibility is particularly important in allowing a stent to be deliverable to very tortuous vessels such as are often found in the heart. The connecting means can attach from a node of one cylindrical segment of stent to a node of another cylindrical segment. The hinged interconnector or the connecting element will allow the stent wall that is on the inside radius of curvature through a tortuous or bent path to compress or contract as the connecting means is deformed to a shorter axial length. The hinge of the hinged interconnector can undergo a plastic deformation or an elastic deformation as the stent passes along a tortuous path. The struts of the hinged interconnector remain elastic during passage along a tortuous path. The connecting means located on the outside of the radius of curvature of a bend is able to extend to a larger length. The connecting means can be formed of the same material as the struts and nodes of the stent and can be machined into the stent structure in a manner similar to the forming of the slots. The radial dimension of the hinged interconnectors can be equal to or smaller than the radial dimension of the struts and can have an equal or smaller width than the struts. The hinged interconnectors and connecting elements are generally designed to remain elastic during the extensional deformation encountered as the stent extends and contracts while extending along a bend in a blood vessel. As the stent in its nondeployed state is bent around a tortuous path the hinged interconnectors and connecting elements provide the stent with a flexible characteristic. After the stent is deployed, the hinged interconnectors also provide the stent with an ability to conform with a tortuous vessel wall without trying to exert forces that could undesirably try to straighten the vessel.
In still another embodiment the stent is formed in an array of nodes and struts wherein each node is connected to three struts. The configuration of nodes and struts can be used for either a balloon-expandable stent or a self-expandable stent. The presence of hinges with a greater radial dimension and a smaller width that the struts provides this embodiment with the advantage of decoupling the inserted stent compression yield force and the implanted stent expansion yield force from the implanted stent crush yield force, and decoupling the implanted and inserted stent expansion elastic force from the implanted stent crush elastic force. In addition, axial flexibility can be provided directly from the array of nodes and struts without the need for connecting means.
In one more embodiment of the present invention the metal tube can be formed into a stent with an array of nodes and struts that have each node connected to two struts. The node can be a single hinge that connects two struts. The hinges and struts can be formed in any combination of straight or curved shape along with any combination of axial, circumferential, or oblique orientation for either the hinges or struts. This stent can be formed into a balloon-expandable stent or a self-expandable stent. In a balloon-expandable stent the hinge uncouples the inserted stent compression yield force and the implanted stent expansion yield force from the implanted stent crush yield force. The strut portion is constructed such that crush deformation will not result in plastic deformation; the strut will remain elastic. This allows the balloon-expandable stent to be balloon-expandable and non-crushable. In a self-expandable stent the hinge uncouples the inserted and implanted stent expansion elastic force from the implanted stent crush elastic force. The strut portion is similarly constructed such that crush deformation will not result in plastic deformation of the strut which will remain elastic. This allows the self-expandable stent to have independent design of inserted and implanted stent expansion elastic force with respect to implanted stent crush elastic force.